Open up the record header for foreign namespace content

[twagoo]

Proposal: open up the header (in the record evelope) for child elements from foreign namespaces.

A concrete use case that I have in mind is adding the OAI-PMH identifier to CMDI records upon harvesting, probably together with some other harvesting related metadata (such as the endpoint, dataset, time of harvesting). This way you can recognise harvested records and see where they came from.

Practically, this would give the VLO (and other aggregators/exploitation tools) an easy way of finding out the record identifier which is currently not (reliably) preserved after extraction and/or conversion. Change would be backwards compatible.

[twagoo]

(also disussed on Slack)